Moment Compensated Bending Beam Sensor for Load Measurement on Platform Supported by Bending Beams
A method is provided for fabricating a bending beam sensor coupled to a touch input device. The method includes providing a bending beam. The method also includes placing a first strain gauge and a second strain gauge on a surface of the beam near a first end of the beam, and aligning the first strain gauge and the second strain gauge with the beam along an axis. The first end is attached to a base. The method further includes coupling the first strain gauge and the second strain gauge to a plate of the touch input device and electrically connecting the first strain gauge and the second strain gauge such that a differential signal is obtained from the first strain gauge and the second strain gauge when a load is applied on the plate of the touch input device.
This Patent Cooperation Treaty patent application claims priority to U.S. provisional application No. 61/642,423, filed May 3, 2012, and entitled, “Moment Compensated Bending Beam Sensor For Load Measurement On Platform Supported By Bending Beams,” the contents of which are incorporated herein by reference in its entirety.
TECHNICAL FIELDThe present invention generally relates to method and sensors for measuring forces on a trackpad or other surface, such as a computing input device.
BACKGROUNDA force sensor, such as a bending beam sensor, measures a force exerted by an object on a surface to which the sensor is affixed. A standard bending beam strain sensor has an output that is large enough to be easily measured with readily available components. The bending beam sensor may include a strain gauge that measures strain generated by the force. Generally, the greater the force, the greater the generated strain. Thus, the strain gauge may be used to indirectly measure and compute the force exerted by a load on the beam, or an object attached to the beam.
However, in some cases a typical bending beam strain sensor may prove inadequate for measuring an exerted force. In the case of a plate supported by multiple beams, for example, it is possible that a force exerted on the plate may be measured inaccurately by a bending beam sensor due to a moment of the force. That is, when a force or load is applied to the plate, the strain measured by the standard bending beam strain sensor depends on the position of the force or load on the trackpad or plate and also the connection between the plate and the bending beam. This may prove problematic in certain mechanisms, such as a force-sensitive input device for a computing device. Examples of such input devices may include trackpads, buttons, keys on a keyboard, pressure-sensitive touch screens and so forth.
Thus, there remains a need for developing methods and device for load measurements on platforms supported with bending beams.
SUMMARYEmbodiments described herein may provide moment compensated bending beam sensors for load measurements on platforms supported by bending beams. The disclosure provides methods for measuring the load on the platforms.
In one embodiment, a method is provided for fabricating a bending beam sensor coupled to a touch input device. The method may include, for example, providing a bending beam, placing a first strain gauge and a second strain gauge on a surface of the beam near a first end of the beam, and aligning the first strain gauge and the second strain gauge with the beam along an axis. The first end typically is attached to a base. The method may employ a first strain gauge and a second strain gauge that are both electrically connected such that a differential signal is obtained from the first strain gauge and the second strain gauge when a load is applied on the plate of the touch input device.
In another embodiment, a method is provided for fabricating a touch input device coupled with bending beam sensors. The method includes providing at least three bending beams. The method also includes placing a first bending beam sensor on a surface of the first bending beam and aligning the first bending beam sensor with the first beam between a first end and a second free end of the first beam, the first end of the first beam attached to a first beam base. The method further includes placing a second bending beam sensor on a surface of the second bending beam and aligning the second bending beam sensor with the second beam between a first end and a second free end of the second beam, the first end of the second beam attached to a second beam base. The method also includes placing a third bending beam sensor on a surface of the third bending beam and aligning the third bending beam sensor with the third beam between a first end and a second free end of the third beam, the first end of the third beam attached to a third beam base. The method further includes coupling the first, second, and third bending beam sensors to a plate of the touch input device and electrically connecting each of the first, second, and third bending beam sensors to output signals to a processor.
In yet another embodiment, a moment compensated bending beam sensor device is provided for a plate coupled to a bending beam. The plate is configured for applying a force on a top surface of the plate. The sensor device includes a bending beam having a first end attached to a beam base. The sensor device also includes a bending beam sensor attached to the bending beam. The bending beam sensor includes a first strain gauge and a second strain gauge, each strain gauge being aligned with the bending beam and being placed between the first end and the second free end.
In still yet another embodiment, a touch input device is provided. The input device includes a plate configured for applying a force on a top surface of the plate. The input device also includes a position sensor attached to the plate. The input device also includes four bending beams coupled to the plate and the position sensor, each bending beam having a first end attached to a beam base. The input device further includes a bending beam sensor attached to each bending beam. Each bending beam sensor includes a first pair of strain gauges and a second pair of strain gauges being aligned with the bending beam and placed between the first end and the second free end, and the first pair of strain gauges and the second pair of strain gauges.
In a further embodiment, a method is provided for determining a force and a location of the force on a plate. The method includes sensing a voltage change at a first strain gauge and a second strain gauge. The first and the second strain gauges are positioned on a common side of a single beam coupled to the plate. The method also includes obtaining a differential voltage between the first strain gauge and the second strain gauge. The method further includes transmitting the differential voltage to a processor; and converting the differential voltage to a force on the plate.
In yet a further embodiment, a touch input device is provided. The input device includes a platform, at least one bending beam supporting the platform. The input device also includes at least one force sensor disposed on the at least one bending beam. The force sensor is operative to measure a force exerted on the platform. The at least one force sensor is operative to output at least three distinct force levels.
Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention. A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings, which forms a part of this disclosure.
The present disclosure may be understood by reference to the following detailed description, taken in conjunction with the drawings as briefly described below. It is noted that, for purposes of illustrative clarity, certain elements in the drawings may not be drawn to scale.
Generally, embodiments discussed herein may take the form of a sensor for determining a load or force, or structures that operate with such sensors. As one example, a trackpad may be associated with one or more force sensor, as discussed herein. As force is applied to the trackpad, the sensor(s) may detect a strain. That strain may be correlated to the force exerted on the trackpad and thus an amount of force exerted may be determined. Further, by employing multiple sensors in appropriate configurations, a location at which a force is applied may be determined in addition to a magnitude of the force.
Some embodiments may permit the detection of multiple forces at multiple locations.
Further, it should be appreciated that such forces may be used as inputs to a computing device. That is, by measuring a force, the force may be treated as an input to a computing system and the input may vary with the degree of force applied. Thus, input/output devices may accept non-binary inputs through the use of strain sensors, as discussed herein. In addition, embodiments may be used in or as a variety of different input/output mechanisms. For example, strain (and thus force) may be measured on a variety of input surfaces in accordance with embodiments discussed herein. Thus, various surfaces may be used as an input to a variety of computing devices, such as laptop and/or desktop computers, smart phones, tablet devices, touch pads, dashboards, control buttons and switches, and so on.
Still with reference to
As shown in
Referring now to
As shown in
In certain embodiments, the support or connection 310 may be a viscoelastic polymer, such as a gel. The term “gel” may refer to any suitable, deformable substance that connects the beam and plate. In some embodiments, an adhesive may be used in place of, or in addition to, a gel. In other embodiments, the gel may be omitted. In still further embodiments, a mechanical fastener may affix the beam and plate.
In
In another embodiment, as shown in
In yet another embodiment, the beam width may be changed to produce a stiffness change. In still yet another embodiment, any combination of the beam thickness variation, beam stiffness variation, beam width change may also create an end substantially stiffer than the beam. In a further embodiment, the beams may have both ends connected to a flexible support or a rigid support. In yet a further embodiment, the two ends of the beam may have a combination of the beam thickness variation, beam stiffness variation, beam width change, which may create two ends substantially stiffer than the beam.
The beam may have a uniform thickness between the two strain gauges 302 and 316. Alternatively, the thickness or width of the beam may change between the two strain gauges. Mathematically, the curvature between the two strain gauges 302 and 316 has a second derivative of zero under an applied load or force. Generally during operation, there are no external loads or forces applied between the two strain gauges.
In one embodiment, the two strain gauges 302 and 316 are connected electrically as one arm of Wheatstone bridge (see
The output voltage for the moment compensated bending beam sensor is a differential signal of the output from the two strain gauges 302 (S1) and 316 (S2). At strain gauge 302,
M1=F(L−x1−a) Equation (1)
ε1=M1t/2EI Equation (2)
dR1=RGε1 Equation (3)
At strain gauge 316,
M2=F(L−x2−a) Equation (4)
ε1=M2t/2EI Equation (5)
dR2=RGε2 Equation (6)
where M1 and M2 are the moments, and ε1 and ε2 are the strains, E is the Young's modulus, I is the moment of inertia of the beam, dR1 and dR2 are the resistance changes of the respective strain gauges 302 and 316, R is the resistance of each of the strain gauges 302 and 316, G is the gauge factor of the strain gauges, t is the thickness of the beam, w is beam width, and L is the length of the beam. a is the position of the force, or the distance of the load from the free end 312 of the beam 306. In some embodiments, the resistances of the two strain gauges may not be equal.
Note that both dR1 and dR2 depend upon the beam length L and the position of the force a. However, a differential signal Δ is independent of the beam length L and the position of the force a. The differential signal is the difference between dR1 and dR2, which is expressed as follows:
Δ=dR1−dR2=RGtF(X2−X1)/2EI Equation (7)
In an alternative embodiment, four strain gauges 302A-B and 316A-B are connected electrically as a full Wheatstone bridge.
The electrical contact 512 may also include a wire bond pad for temperature compensation. Again, the carrier with the strain gauges S1A, S1B, S2A, and S2B is aligned with the central X-axis of the beam.
Aluminum and steel are popular choices for a beam material. They are commonly available in many useful preformed sizes and strain sensors are available with built in compensation for thermal expansion. Other materials are possible, including titanium, plastic, brass and so on.
Additionally, this disclosure provides a method for implementing a plate mounting scheme, where the plate is supported on its four corners by four bending beams. The plate is attached to the beams in any suitable fashion, such as by a viscoelastic polymer. In alternative embodiments, the plate may be attached to the beams with adhesive, through welding, mechanical fixtures and the like.
Each of the four bending beams has a bending beam sensor including strain gauges. The gel 310 may exhibit a viscoelastic response and change shape in response to the applied force with a time constant of seconds. As the gel changes shape, the location of the applied force shifts. Because the strain gauges are moment insensitive, the outputs of the strain gauges are not affected by this viscoelastic response of the polymer.
Each moment compensated beam sensor includes at least two strain gauges which are wired together to produce a differential signal in one embodiment. In an alternative embodiment, each moment compensated beam sensor includes four strain gauges which can be wired as a Wheatstone bridge. For the plate, load signals can be obtained from the bending beam sensors in order to determine the force exerted on the trackpad, and load position signals can be obtained from the position sensors.
In a particular embodiment, the bending beam may be approximately 10 mm wide, 10 mm long and 0.5 mm thick, and the trackpad may be approximately 105 mm long and 76 mm wide with thickness ranging from 0.8 mm to 2.3 mm.
It will be appreciated by those skilled in the art that the dimension of the beam may vary for various desired loads and electrical outputs as well as the dimension and shape of the platform.
In certain embodiments, a position-sensing layer may underlie the plate. The position-sensing layer may be, for example, a capacitive sensing layer similar to that employed by many touch screens. The capacitive sensing layer may include electrodes arranged in rows and columns and operative to sense the particular position of a touch. In some embodiments, the position-sensing layer may sense multiple simultaneous touches in a fashion similar to that of a touch screen incorporated into a smart phone, tablet computing device, media player, and like products. As the operation of the touch-sensitive layer is known in the art, it will not be discussed further herein.
It should be appreciated, however, that the position sensing and force sensing of the trackpad may be combined. Accordingly, the various discussions herein regarding force sensing may be applied to a capacitive sensing layer and/or a capacitive sensing display, as well as any other computing element or enclosure that may be touched, pressed or otherwise interacted with. Embodiments described herein may be configured such that forces applied to a display or other computing element may be sensed. The trackpad plate may be replaced by a cover glass or surface of a mobile device, or the like, and forces on such a surface sensed.
In a particular embodiment, the beam has a uniform thickness to reduce the overall dimensions of the trackpad. For certain applications, such as in a tablet computing device, media player, portable computer, smart phone, and the like, a connection between the plate and the beams through a viscoelastic polymer, such as a gel, would be thin.
In some cases, it is desired to approximately determine the force location without using the position sensor or position-sensing layer 710. For each moment compensated beam sensor, the force detected by the beam sensor is multiplied by the position along the central axis of the beam that the force is applied to the individual beam forming a force distance product. The force distance products of all four beams are summed and divided by the total force. The resulting position approximates the position of the force relative to the center of the trackpad. Essentially, the use of three beam sensors permits triangulation of the location of a force by comparing the relative magnitudes of the forces sensed by each beam sensor, although four bending beams are shown in
Further, in the case of multi-touch gestures, the location and magnitude of multiple forces may be determined from the outputs of the position sensor and the bending beam sensors, each load correlated with a different touch on the trackpad or other input mechanism. For example, when using two or more fingers to touch a track pad simultaneously, it is required to determine the location and magnitude of multiple forces.
A moment compensated bending beam sensor may be used for both relatively thin platforms, such as those approximately 0.8 to 1.0 millimeters thick or less, and relatively thick platforms. “Relatively thick,” as used here, refers to platforms having a thickness approximately equal to, or greater than, 2.3 millimeters Some examples are shown below.
The moment compensated bending beam sensors may include one or more strain gauges to measure force. The position sensors 1604 may include capacitive measuring electrodes. The trackpad is a touch input device which is different from a simple binary mechanical switch, which may be in an “on” or “off” state. The touch input device can measure a variable force or a constant force and output more than “over threshold” or “under threshold”. The platform may be optically transparent or opaque.
It should be appreciated that the present embodiment employs a double bending beam strain gauge but does so on a non-standard beam. That is, the beam itself is not a double-bending (or contraflexured) beam. In contrast to double bending beams, neither the angle of the beam 306 at its root or the angle of the beam at the free end are constrained to be fixed or parallel. The beam largely deforms along a single curve when a force is applied instead of bending into an S-shape like a double-bending beam. Further, unlike many contraflexured beams, the present beams may have a relatively uniform thickness. Many contraflexured beams are thinner in cross-section at one point along their length to induce the S-shape curvature when the beam is loaded. In an alternative embodiment, the beam thickness may vary. For example, the beam thickness in the strain gauge area or an active area may vary from a non-active area without the strain gauge. Still further, some embodiments discussed herein generally place all strain gauges on a single side of each beam rather than distributing them across opposing sides as may be done with both contraflexure beams and single-bending beams. In this invention, the strain sensors have been described as resistive gauges in which the resistance is proportional to the beam strain. It will be recognized by those skilled in the art that semiconductor strain gauges, micromachined strain gauges or optical strain gauges could also be employed in a similar fashion to provide a signal that is independent of the load position.
Moreover, the signals from the differential strain gauges 302 and 316 may be combined in a Wheatstone bridge; however, in some instances, it may be desirable to convert the electrical signals from the differential strain gauges separately into digital form. These digital signals could then be scaled and subtracted to provide a moment compensated signal. Independent scaling of the two gauge signals may be especially desired when the thickness of the beam varies between the location of strain gauge 302 and strain gauge 316.
Having described several embodiments, it will be recognized by those skilled in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention.
Those skilled in the art will appreciate that the presently disclosed embodiments teach by way of example and not by limitation. Therefore, the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween.
Claims
1. A method for fabricating a bending beam sensor coupled to a touch input device, the method comprising:
- providing a bending beam;
- placing a first strain gauge and a second strain gauge on a surface of the beam near a first end of the beam;
- aligning the first strain gauge and the second strain gauge with the beam along an axis, the first end being attached to a base; and
- coupling the first strain gauge and the second strain gauge to a plate of the touch input device; wherein the first strain gauge and the second strain gauge are electrically connected such that a differential signal is obtained from the first strain gauge and the second strain gauge when a load is applied on the plate of the touch input device.
2. The method of claim 1, further comprising attaching a bottom surface of the plate to the top of the beam at the second free end by using a viscoelastic polymer.
3. The method of claim 1, wherein the beam is configured to bend near the second free end of the beam such that the second free end is angled from the first end of the beam.
4. A method for fabricating a touch input device coupled with bending beam sensors, the method comprising:
- providing at least three bending beams;
- placing a first bending beam sensor on a surface of the first bending beam;
- aligning the first bending beam sensor with the first beam between a first end and a second free end of the first beam, the first end of the first beam attached to a first beam base;
- placing a second bending beam sensor on a surface of the second bending beam;
- aligning the second bending beam sensor with the second beam between a first end and a second free end of the second beam, the first end of the second beam attached to a second beam base;
- placing a third bending beam sensor on a surface of the third bending beam;
- aligning the third bending beam sensor with the third beam between a first end and a second free end of the third beam, the first end of the third beam attached to a third beam base; coupling the first, second, and third bending beam sensors to a plate of the touch input device; and
- electrically connecting each of the first, second, and third bending beam sensors to output signals to a processor.
5. The method of claim 4, further comprising:
- attaching a second sensor for position to a bottom surface of the plate; and
- attaching the second sensor for position to the top surface of each beam at the second free end by using a viscoelastic polymer, wherein the first end is attached to a beam base.
6. The method of claim 4, the operation of placing a first bending beam sensor on a surface of each beam comprising:
- placing a first pair of strain gauges on a surface of each beam near one end of the beam;
- placing a second pair of strain gauges on the surface of the beam near a free end of the beam; and
- electrically connecting the two pairs of strain gauges as a Wheatstone bridge such that the first bending beam sensor outputs a signal when a load is applied on the plate.
7. The method of claim 4, wherein each of the first, second, and third bending beams is configured to bend near the second free end of the beam such that the second free end is angled from the first end of the beam.
8. A moment compensated bending beam sensor device for a plate coupled to a bending beam, the plate configured for applying a force on a top surface of the plate, the sensor device comprising:
- a bending beam having a first end attached to a beam base; and
- a bending beam sensor attached to the bending beam, wherein the bending beam sensor comprises a first strain gauge and a second strain gauge, each strain gauge being aligned with the bending beam and being placed between the first end and the second free end.
9. The sensor device of claim 8, wherein the first strain gauge and the second strain gauge are electrically connected to generate a differential output when the force is applied on the plate.
11. The sensor device of claim 8, wherein the bending beam is coupled to the plate through a viscoelastic polymer near a second free end of the beam, the bending beam sensor being at a further distance from the second free end than the viscoelastic polymer.
12. The sensor device of claim 8, wherein the bending beam sensor comprises a carrier, the first strain gauge and the second strain gauge being placed on the carrier.
13. The sensor device of claim 8, wherein the beam is configured to bend near the second free end of the beam such that the second free end is angled from the first end of the beam.
14. A touch input device, the device comprising:
- a plate configured for applying a force on a top surface of the plate;
- a position sensor attached to the plate;
- four bending beams coupled to the plate and the position sensor, each bending beam having a first end attached to a beam base; and
- a bending beam sensor attached to each bending beam, wherein each bending beam sensor comprises a first pair of strain gauges and a second pair of strain gauges being aligned with the bending beam and placed between the first end and the second free end, and the first pair of strain gauges and the second pair of strain gauges.
15. The device of claim 14, wherein the first pair of strain gauges and the second pair of strain gauges are electrically wired as a Wheatstone bridge such that the each bending beam sensor outputs a signal when the force is applied.
16. The device of claim 14, wherein the bending beam sensor comprises a carrier, the first pair of strain gauges and the second pair of strain gauges being placed on the carrier.
17. The device of claim 14, wherein each beam is configured to bend near the second free end of the beam such that the second free end is angled from the first end of the beam.
18. A method for determining a force on a plate, the method comprising:
- sensing a voltage change at a first strain gauge
- and a second strain gauge, the first and the second strain gauges are positioned on a common side of a single beam coupled to the plate;
- obtaining a differential voltage between the first strain gauge and the second strain gauge;
- transmitting the differential voltage to a processor; and
- converting the differential voltage to a force on the plate.
19. The method of claim 18, further comprising determining the location of the force.
20. A touch input device, comprising:
- a platform;
- at least one bending beam supporting the platform; and
- at least one force sensor disposed on the at least one bending beam, the force sensor operative to measure a force exerted on the platform, wherein the at least one force sensor is operative to output at least three distinct force levels.
21. The touch input device of claim 20, further comprising a position sensor coupled to the platform.
22. The touch input device of claim 20, wherein the at least one force sensor comprises at least one strain gauge, and the position sensor comprises capacitive measuring electrodes.
Type: Application
Filed: Mar 15, 2013
Publication Date: Apr 30, 2015
Patent Grant number: 10088937
Inventors: Storrs T. Hoen (Cupertino, CA), Peteris K. Augenbergs (Cupertino, CA), John M. Brock (San Carlos, CA), Jonah A. Harley (Cupertino, CA), Sam Rhea Sarcia (Cupertino, CA)
Application Number: 14/398,720
International Classification: G06F 3/041 (20060101); G06F 3/044 (20060101); H05K 13/00 (20060101); G01L 1/22 (20060101);